I have installed skype in ubuntu but duuno where to start the installed the skype application .Similarly I have installed netbeans and many applications but after installing them their icon's are not appearing in any menus.

There's still a set of 'standard' locations that programs are put into. For example, binaries (in windows parlance 'executables') are put in /usr/bin, and documentation is put in /usr/share/doc in a Ubuntu system.

This is a pretty simplistic answer. For more details about how linux/unix directories are typically structured, see this:

But the general idea is that the /bin and /usr/bin are where the start up application files (executables or scripts) are stored. The direct equivalent of "Program Files" though is probably /usr/share this is the directory that contains the various support files for most applications

For a good overview of the Ubuntu file structure check out the link below.

There is no "Program Files" Directory in Linux. The Application's Parts are distributed according to what they are. There are a lot of explanations of the Directory Structure out there, you can use this one for a start.